I pulled this engine because I was replacing the floor in my boat. The engine ran great, had an 8 psi split between the highest and lowest cylinders in a compression test, and all cylinders passed the leakdown test. Since I pulled the engine, I decided to give it a quick look to see if it needed any attention.

After cleaning a lot of carbon off the pistons, I found two pistons that had some damage. Are these pistons useable? If not, can I just replace the two pistons or do I have replace them all?

Interesting....
Top pic looks like some sort of solid small object made dimples in the piston. Second one looks like some sort of erosion caused by detonation or coolant/water.
While there may be some irregularities on the tops of the pistons, it really is the damage and wear to the sides of the pistons and the condition of the cylinder block bores.
Does the top ring of the piston(s) rotate freely? Any other abnormal marks in the bores? Any marks in the cylinder heads (of the corresponding cylinders?). If not, those pistons are re-useable. Any other suggestions would require additional information.

You already have the heads and pan off, so how far are you going? If the engine ran fine and compression was good, what made you decide to go after the internals of a good engine? Might as well rebuild now....

Well I pulled this engine apart because it was leaking a significant amount of oil and has roughly 1000 hours on it (to the best of my knowledge). After 2-3 days, there would be pooled oil in the bilge. I don't really know the history of the boat but it's had some neglect for sure. So between those three things, I figured the engine could use some lovin'.

The condition of the heads are fine and the ring gaps in the pistons appear to be undamaged. The cylinder bores aren't perfect but a good hone should clean most everything up. The taper isn't too bad. The worst cylinder bores aren't from these pistons though. My plans was to find the source of the oil leak and certainly replace the oil pump while I'm in there. The rod bearing were ok but the crank bearings were gouged up some. The rods and the crank still look decent though but I haven't measured them yet.

I was curious as to what my ring gap was after all these years so I measured 4 rings in their corresponding cylinders...I didn't think about it for the first 5 cylinders. Anyway, the cylinder with the detonation looking marks on it had a 1/4"-1/2" piece of one of the rings broken off. Yet that wasn't the worst cylinder in the compression test.

Here is a picture of the side of that piston with the detonation looking marks on it.

Well I pulled this engine apart because it was leaking a significant amount of oil and has roughly 1000 hours on it (to the best of my knowledge). After 2-3 days, there would be pooled oil in the bilge. I don't really know the history of the boat but it's had some neglect for sure. So between those three things, I figured the engine could use some lovin'.

The condition of the heads are fine and the ring gaps in the pistons appear to be undamaged. The cylinder bores aren't perfect but a good hone should clean most everything up. The taper isn't too bad. The worst cylinder bores aren't from these pistons though. My plans was to find the source of the oil leak and certainly replace the oil pump while I'm in there. The rod bearing were ok but the crank bearings were gouged up some. The rods and the crank still look decent though but I haven't measured them yet.

I was curious as to what my ring gap was after all these years so I measured 4 rings in their corresponding cylinders...I didn't think about it for the first 5 cylinders. Anyway, the cylinder with the detonation looking marks on it had a 1/4"-1/2" piece of one of the rings broken off. Yet that wasn't the worst cylinder in the compression test.

Here is a picture of the side of that piston with the detonation looking marks on it.

Well you found what dinged up the top of piston right there.

I wouldn't reuse any of those pistons at 1000 hours. If you are going to rebuild it might as well go 30 over and get the block cleaned up. You should probably go 10/10 on the crank too.

Ok well the rod, crankshaft, and cam are within spec with room to spare so I don't really see any point in touching those. Ring gaps after honing are around 0.025" on the first ring in the middle of the bore which I can see being a hair big for a mostly stock engine but would that really justify an overbore?

I luv the OCD... since I pulled the engine to fix the floor..I decided to disassemble the engine... If you know what your doin' it probably doesn't seem like a big deal...I for one am impressed and admire with those who have the confidence to rip down an engine and then put it back together... let's see the man cave where the work is being done

__________________...A bad day water skiing still beats a good day at work...1995 Pro Star 205....